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Amazon's ELB looks interesting, but I have a hard time evaluating how reliable it might be. I don't have a good sense of how it's implemented or what it might do behind the scenes.

If an ELB machine goes down that is proxying a request to one of my EC2 isntances, I expect that request to die. What about the next request that client makes? It had resolved an IP that used to pointed to some ELB endpoint that was on a machine that crashed. Is there a good chance the next few requests fail too? At this point are we at the mercy of a DNS TTL expiration and update?

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Adding information about other implementations, here is the design of Eucalyptus ELB. –  Lourenco Jun 24 at 13:48

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ELB instances use an elastic IP which won't change if an ELB backend fails, so DNS isn't an issue here. Amazon don't publish how ELB actually works though, so what exactly happens when a backend fails is pure speculation. I would guess that failover time is in the order of seconds however.

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Amazon have very good documentaion on AWS ELB below is the link and also they guarantee 99.99 % uptime for AWS ELB

  http://aws.amazon.com/elasticloadbalancing/
  http://aws.amazon.com/documentation/elasticloadbalancing/
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Neither of those links explain how Amazon ELB is implemented under the hood. –  Simplicius May 16 '13 at 5:32
    
Amazon have never and never will telk how there backends work. –  t1nt1n May 16 '13 at 5:47

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